1. Field of the Invention
This invention relates to desulfurization, and in particular to simultaneously pretreating a hydrocarbon feedstock for reforming and conversion thereof to isobutane.
2. Description of the Prior Art
Naphtha stocks, e.g. napthas of C.sub.6 -400.degree. F., are generally treated in refinery operations by catalytic reforming in which the naphthenes contained in the naphtha are dehydrogenated to aromatics and cyclopentanes are isomerized to cyclohexanes. Some cracking of naphthenes occurs under controlled reaction conditions. The typical catalyst used in reforming is platinum on an alumina base (U.S. Pat. Nos. 3,110,703 and 2,772,217) the platinum acting as a dehydrogenation component; platinum-rhenium catalysts have also been used. The main hydrocarbon product from the reformer is combined with hydrocarbon products of other operations to also provide high octane gasoline products. A low pressure hydrogen is a by-product.
However, naphtha feedstocks may contain high levels of sulfur or sulfur compounds to seriously affect the reforming catalyst. Reformer feeds are therefore usually subjected to a desulfurization step in the presence of hydrogen, termed "pretreating" (U.S. Pat. No. 2,724,683). U.S. Pat. No. 3,743,594 discloses such a process using a Group VIII metal component on a "layered" crystalline aluminosilicate to prevent hydrocracking. Crystalline zeolitic alumiosilicates are said to result in excessive hydrocracking. U.S. Pat. No. 2,885,352 also describes hydrodesulfurization; hydrocracking of a desulfurized gas oil and other stocks is discussed.
Hydrocracking is a known operation which involves cracking a hydrocarbon feedstock to smaller molecules in the presence of hydrogen. Stocks boiling in the range of 400.degree. to about 1100.degree. F, such as virgin heavy, light vacuum and coker gas oils, gas oil from cracking processes, furfural extracts and mixtures of these have generally been cracked under pressures ranging from 1000 to 3000 psig.
Hydrocracking catalysts may include crystalline aluminosilicate zeolites in a matrix (U.S. Pat. No. 3,702,818) and contain a hydrogenation component, usually a Group VIII metal. A very desirable product from the hydrocracking step is isobutane which, when combined with olefins in an alkylation process, can produce high octane gasolines.
Naphtha stocks have been subjected to hydrocracking (U.S. Pat. No. 3,579,434) at 2000 psig using a palladium on chelated aluminosilicate zeolite Y which had been treated with dimethyl sulfoxide (DSMO), with conversions to C.sub.1 -C.sub.5 of 39% to 70% by volume. Chelation is taught therein as a required treating step. There is no indication that sulfur is removed. The hydrogen circulation was 7500 scf/b of naphtha.
U.S. Pat. No. 3,427,243 discloses hydrocracking with noble metal-containing zeolites. The pressures required are above 500 psig.
Other patents of interest are U.S. Pat. Nos. 3,458,433 and 3,781,199.